A number of issues affect how well different tears heal -
The meniscus is nourished from a network of small blood vessels that penetrate from the outer meniscal rim. Here on the outer rim the blood supply is rich and healing is generally good. In this area tears are referred to as 'red-on-red' implying that both sides of the tear have a good amount of blood vessels.
On the inner thin edge of the meniscus, however, the blood vessels do not penetrate at all. The cells of the meniscus in this region are dependent on any nourishment they can get from the joint fluid. Tears here do not heal. In this area tears are referred to as 'white-on-white' implying a lack of blood vessels on both sides of the tear.
Tears in-between these two regions are referred to as 'red-on-white'. They may heal, but not necessarily.
Without treatment, tears tend to get bigger rather than healing themselves. Once there is a tear in the meniscus, stability of the joint is altered and the forces on the meniscus itself alter, and the tear is likely to propogate and get larger. The larger the tear gets, the greater the alteration in joint mechanics.
Some anatomical areas are harder to treat than others. Special attention is given to tears of the front and rear parts of the meniscus - the anterior and posterior horns. Tears in both areas are frequent and more difficult to manage than tears in the middle of the wedge. Horizontal tears are also a problem - they tend to extend deep into the substance of the meniscus and it is hard to trim to healthy areas without going right to the all-important rim, which is a critical element of the meniscus.